Comment made by: alex+import
(I assume there's a typo in the description - 123 is returned, not 1232)
It's not just literal values, non-whitespace at the end of any input is silently ignored and should be rejected:
(json/read-str "{}xxx") => {}
(json/read-str "[]yyy") => []
(json/read-str "\"\"zzz") => ""
NB This behaviour agrees with the docstring ("Reads a single item of JSON data from ...").